Substance R is red in solution, whereas substance S is blue in solution.
R(aq)⇌S(aq) \mathrm{R(aq) \rightleftharpoons S(aq)} R(aq)⇌S(aq)The forward reaction is exothermic. An equilibrium mixture in a sealed tube appears purple at room temperature. Without opening the tube, a student warms the mixture in a hot-water bath.
What is observed?
The solution changes towards blue because warming favours the forward, exothermic reaction.
The solution becomes darker purple because both reactions continue while equilibrium is restored.
The solution changes towards red because warming favours the reverse, endothermic reaction.
The solution stays purple because equilibrium fixes the amounts of both substances.
124 exam-style questions on OCR GCSE Chemistry 5.3 Equilibria, covering 5.3.1 Reversible reactions and dynamic equilibrium and 5.3.2 Changing the position of equilibrium. Each one has a worked solution and a mark scheme showing where the marks go.